The present invention relates to the metal processing arts. It finds particular application in conjunction with mechanical equipment for moving or pumping metals, such as magnesium, aluminum, and zinc, in a bath of molten metal, and will be described with particular reference thereto. An alloy is provided which is particularly suited to forming components for use in magnesium and magnesium/aluminum baths for refining of magnesium, at temperatures up to around 1800° F. Such components include, for example, pumps, tubing, ladles, troughs, rolls, drivers, furnace, equipment, risers, and the like. It should be appreciated, however, that the invention is also applicable to a variety of metal processing industries in which the processing equipment is submerged in a bath of molten metals.
Baths of molten metals, such as magnesium, zinc, and combinations of magnesium and aluminum or zinc and aluminum are widely used in the metal processing industries. To retain the metals in their molten state, bath temperatures of up to 1300° F. for zinc-rich baths and 1800° F. for magnesium-rich baths are typically encountered.
Equipment used for moving and transferring metals in a bath of molten metal, conventionally have a relatively short life because of the destructive effects of the molten metal on the components contacting the molten metal. Pump shafts, for example, connecting a motor to an impeller, are often formed of steel to provide sufficient torque to move the impeller and the molten metal. Such shafts tend to have a short life because the steel is chemically attacked by the molten metal. If the steel shaft is shielded by a protective coating of a ceramic material, the different thermal-expansion characteristics of the steel and the ceramic tend to cause the ceramic to shatter in a relatively short time. Parts made of graphite rather than steel tend to burn at the metal surface. Pure ceramic components do not have sufficient tensile torque or impact strength to overcome the stresses normally encountered when utilized in molten metals.
Hot dip metalizing coating processes for galvanizing steel used in the automotive, construction, and appliance industries also require equipment that runs submerged in a molten metal. Strips of steel to be galvanized are passed around a sink roll in a molten bath of zinc [galvanizing], aluminum [aluminizing], or aluminum-zinc [galvanneal] in which the levels of aluminum vary from a fraction of a percent to as much as 100 percent. The molten metal temperature is of the order of 820° F. to as high as 1300° F.
Standard rolls and equipment used in magnesium processing, where the molten metal is 95-100% magnesium, are generally formed from cast iron or mild steel. Pumps and bearings, in particular, require continuous replacement and maintenance. Often, the equipment components are removed and replaced weekly, and in some cases, daily. Cast iron and mild steel are not formulated specifically for these applications and consequently lack the properties to meet the operational needs. A suitable alloy for fabrication of molten metal equipment, such as power-driven pumps and bearings should have the following desirable characteristics:                1. Low solubility in the molten magnesium. In other words, a material loss of less than 10−7 cm/hour.        2. Low adhesion (non-wettable) to Mg and Mg salts and dross. Wetting plays the main role in the bonding of solid-liquid state metals.        3. High surface hardness (Rc larger than 30). Abrasive wear contributes nearly half of the loss of bearing life in smelting applications.        4. Dimensional stability at operating temperatures of up to 1500° F. for straightness and roundness. Pumps that operate over 1000 RPM tend to generate excessive vibration and damage to the bearings and holding equipment when dimensional stability is low.        5. Thermal shock resistance. The equipment should be capable of withstanding a shock of no less than 700° F. when going from air to molten metal and 1400° F. when going from molten metal to air.        6. Good impact and notch resistance strength.        7. Castable and machinable by standard procedures to provide simple and available maintenance.        8. Tensile strength and elastic modulus compatible with the application.        
The present invention provides a new and improved alloy suited to used in advanced molten magnesium handling equipment, galvanizing equipment, and other equipment to be submerged or partially submerged in molten metals, which overcomes the above-referenced problems and others.